Cyclohexyl xanthates

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Scheme 4.3 Propagation steps for the reaction of cyclohexyl xanthate with (TMS)3 SiH...

The same products, but in different ratios, occur in the gas phase eliminations of HCl from bornyl and isobornyl chlorides and from the pyrolysis reaction of bornyl and isobornyl benzoates and methyl xanthates . The isobornyl reaction is appreciably faster than that of the bornyl ester ((iso-B/B) = 6.8 at 345 °C), but proceeds at a slightly slower rate than that of cyclohexyl acetate, (CH/iso-B) = 2.1 at 600 °K. By analogy with the nonclassical carbonium ion interpretation of the solvolysis rates of bornyl and isobornyl chlorides, the participation of nonclassical carbonium ion intimate ion-pairs, e.g. 

The methyl-tranj-2-p-toluenesulfonylcyclohexyl sulfite similarly decomposes by cis elimination. In contrast, the irons elimination observed in the methyl-m-2-/i-toluene sulfonyl cyclohexyl sulfite is believed to occur by way of a dipolar ion intermediate, similar to that used to explain irons elimination from the corresponding xanthate, viz. 

Accelerators used include hexamine, diphenyl guanidine, ethylidene aniline, mercapto-benzothiazole, dibenzothiazole disulfide, N-Cyclohexyl benzothiazole sulfenamide, sodium diethyl dithiocarbamate, tetramethylthiuram disulfide, tetraethyl thiuram disulfide, dipentamethylene thiuram tetrasulfide, sodium isopropyl xanthate, zinc butyl,... 

PRODUCTS OF PYROLYSIS OF cis- AND Ira i-2-S U BSTIT UTE D CYCLOHEXYL ESTERS AND XANTHATES... 

For four- or five-membered cyclic transition states, pyrolysis only leads to beta-elimination if the C -H and C -X bonds can eclipse so as to ensure the necessary proximity for reaction. The syn nature of the amine oxide decomposition has been demonstrated by elimination from the diastereoisomeric pairs of the 1,2-diphenylpropyl system. In the cyclohexyl series, an eclipsed transition state is only achieved if elimination proceeds through the boat conformer and consequently the pyrolysis of 1-methylcyclohexylamine oxide gives an almost quantitative yield of the less stable exo-olefin. This orientation contrasts markedly with that of the six-membered cyclic transition states of the corresponding esters and xanthates which yield predominantly the endo-olefin, syn-clinal stereochemistry being preferred. For all the other ring... 

N-Cyclohexyl-2-benzothiazolesulfenamide 1,3-Dibutylthiourea Dicyclohexylbenzothiazyl-2-sulfenamide Dimethyl diphenyl thiuram disulfide 2,6-Dimethylmorpholine Diphenylamine 4,4 -Dithiodimorpholine 2-Ethylbutyraldehyde Ethyl morpholine Formaldehyde aniline 2,6-Lutidine N-Nitrosodimethylamine Paraldehyde p-Quinone dioxime Selenium Selenium dimethyidithiocarbamate Sodium diethyidithiocarbamate Sodium dimethyidithiocarbamate Sodium nitrite N,N,N, N -Tetrabenzylthiuram disulfide Tetraisobutylthiuram disulfide Tetramethylthiuram disulfide Thiocarbamyl sulfenamide Thiophenol Thiourea Triethanolamine Triethylamine Triethylenetetramine n-Valeraldehyde Zinc dialkyl dithiophosphate Zinc diamyidithiocarbamate Zinc dibenzyl dithiocarbamate Zinc diisobutyidithiocarbamate Zinc isopropyl xanthate Zinc 2-mercaptobenzothiazole Zinc oxide Zinc-N-pentamethylene dithiocarbamate Zinc peroxide accelerator, rubber articles for repeated use food-contact... 

X-xan = 0-substituted xanthate) in benzene at 25°C. This reaction represents the addition of a bidentate ligand to give an octahedral complex. The latter presumably has the cis configuration so a substitution process is involved in the reaction. Values of kf and K are dependent upon the nature of the substituent X, as are the equilibrium constants (calculated from kfikr as they are too large to be measured directly with accuracy). For the reaction in which X = cyclohexyl the activation parameters are A/// = 5.4 0.6 kcal mol = 24 1 kcal mol-, Sf = -21 4 cal K mol and A5f = 26 3 cal K" moF. These values reflect the nature of the processes involved and are similar to values found for analogous reactions of Ni(II) substrates when steric factors were absent. The substituent effects are discussed in terms of Taft s a parameters. Reactions... 

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